Electrical stimulation of tissues is an increasingly valuable tool for treating a variety of disorders. Electrical stimulators have many applications, such as cochlear implants for use in treating profound hearing loss, visual prostheses for treating blindness, spinal cord stimulators for treating severe chronic pain, muscle stimulators for treating paralysis, cardiac pacemakers for treating a variety of cardiac ailments, and deep brain stimulators for treating a number of neurological disorders. Deep brain stimulation, for example, can be used to treat tremor and Parkinson disease, and has also shown potential benefit for treating a variety of other disorders such as Tourette syndrome, pain, depression, and obsessive-compulsive disorder. Brain implants for paralysis treatments are increasingly providing sensory feedback via neural stimulation.
In the majority of implementations, clinicians make an effort to avoid the need to extend implantable electrical stimulators through the skin (for example, to connect the device to an external energy source) due to risk of infection. As such, these stimulators are often powered by an implanted battery or by an implanted RF coil receiving energy wirelessly. Thus, the energy efficiency of the stimulator is important in determining the size of the battery or coil and the lifetime of the device, and improvements in stimulator energy efficiency lead directly to reductions in battery or coil size, increases in battery lifetime, and reductions in tissue heating. Patient safety and comfort are increased, and medical costs are reduced if the size of the implant can be decreased.
In existing applications, current-source-based stimulators are generally favored because of their safety, established methods of charge balancing, and overall facility of implementation. But current-source stimulators are inefficient, consuming up to ten times the energy necessary to achieve stimulation of tissue. Voltage-based stimulators are sometimes used as an alternative to current stimulators due to their inherently higher energy efficiency. But voltage-based stimulators suffer from poor charge and current control and are sensitive to changes in electrode impedance.
Some stimulator systems have been developed that use a network of capacitors pre-charged to specific voltages as a power source for the electrode. The capacitors are connected directly to the electrode in sequence as a means of keeping the difference between the electrode voltage and capacitor voltage small, thus enabling improvements in energy efficiency. In those systems, though, currents are neither constant nor controlled.
Alternative systems have been developed that use added current limiters in series with the electrode to keep current through the electrode relatively constant. Because capacitor banks only allow for coarse discrete operation, though, both these implementations are inherently less energy efficient than continuous voltage-based implementations. The use of explicit current limiters in these implementations degrades energy efficiency.